[Tutor] variable inheritance

[spir]

Hello,

Say I have a Tree type that may be based on 2 kinds of Node-s. Both conceptually and practically, a tree itself is a node, namely the top/root one. But a tree also has some additional tree-level behaviour, which is independant of the kind of node. Say, the node type performs all the underlying tree mechanics. Thus the Tree type looks like:

class Tree(_Node):
    def __init__(self, args):
        _Node.__init__(self)
        ...
    ... tree-level methods ...

The issue is the actual kind of node is not only initialised through "_Node.__init__(self)": it must first feature as super class in "class Tree(Node)". (Otherwise I get an error about wrong type of self.)
I tried to play with __new__ in Tree and Node types, but this error seems unavoidable. Maybe I did not do it right. I can indeed have __new__ return an instance of either Node type, but then for any reason it "forgets" it is also a tree (so that calling any tree method fails).
I there a way to do this right?

Workaround ideas:
* Make the root node an attribute of tree: unsatisfying.
* Create a common tree type and 2 specialised ones: 
class TreeN(Tree,NodeN)
    def __init__(self, args):
        _NodeN.__init__(self)
        Tree.__init__(self, args)
# ... e basta ...


Denis
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